Overview
IEC/IEEE 61869-21:2025 - "Instrument transformers - Part 21: Uncertainty evaluation in the accuracy test of instrument transformers" - defines requirements, methods and guidance for evaluating measurement uncertainty when testing the accuracy of instrument transformers (ITs). It applies to ITs with analogue or digital secondary signals used for measuring, protection and control at rated frequencies from 15 Hz to 400 Hz, and is independent of technology (inductive or non‑inductive). The standard explains how to identify uncertainty sources in test setups and how to combine their effects to produce a defensible uncertainty budget for accuracy test results, including on‑site testing.
Key topics and requirements
- Scope and applicability: ITs with analogue or digital outputs; lab and on‑site accuracy verification; 15 Hz–400 Hz rated frequencies.
- Sources of uncertainty: reference IT, comparator, burden, voltage/current source, and setup‑specific contributions.
- Requirements for test equipment: minimum characteristics and performance expectations for reference ITs, comparators and burdens used in accuracy tests.
- Measurement model and uncertainty budget: preparation of measurement equations, identification and quantification of input quantities, and combination of uncertainty contributions.
- Procedures and methods: guidance on data acquisition (sampling frequency, time intervals), connections, demagnetization of inductive CTs, application of Type A (statistical) methods, and special applications.
- On‑site testing guidance: objectives, intervals, additional uncertainty sources and adaptations for reduced‑amplitude tests.
- Informative annexes: example uncertainty calculations and linearly‑reduced test approaches for on‑site verification.
Practical applications and users
Who will use this standard:
- Test laboratories performing type and routine accuracy tests of CTs and VTs.
- Calibration houses developing uncertainty budgets and calibration certificates.
- Field engineers and utilities conducting on‑site verification and acceptance testing.
- Manufacturers of instrument transformers and digital comparators preparing test procedures and technical documentation.
Practical benefits:
- Provides a consistent, traceable approach to quantify and report uncertainty in IT accuracy tests.
- Improves comparability of results across labs and field checks.
- Supports conformance assessment, procurement specifications, and risk‑based decision making for protection and metering systems.
Related standards
- Other parts of the IEC 61869 series (Instrument transformers) for performance and testing requirements.
- Measurement and calibration best practices and guides for constructing uncertainty budgets (refer to national/international metrology guides).
Keywords: IEC/IEEE 61869-21:2025, instrument transformers, uncertainty evaluation, accuracy test, on‑site testing, comparator, reference IT, burden, measurement uncertainty, digital comparators.